Large area hybrid microcircuit assembly

ABSTRACT

A ceramic substrate includes a multiplicity of alternately screened and fired conductor and dielectric levels, with interconnections between levels through small via holes in the dielectric layers or alternately by means of crossovers. Thick film resistors are incorporated throughout the ceramic substrate, wherever needed, and the thick film resistors are printed and trimmed through windows which extend through any upper layers. In addition, resistors may be printed directly on top of the dielectric layers where necessary. Selective area hermetic sealing is utilized only for those chips and wire bonds which require environmental protection by placement of a ring frame seal encircling the selected area and over insulation on substrate conductors which are routed directly beneath the sealing area. A lid is directly soldered to the frame and, before final sealing, a proper environment is created within a sealed area. Where equivalent chip versions of electronic components are not available, discrete components are soldered directly to the ceramic substrate outside of the sealed area. Necessary thermal dissipation can be afforded by direct mounting of the multilevel substrate on a heat sink.

CROSS REFERENCE TO RELATED APPLICATION

This is division of copending Ser. No. 104,471, filed Dec. 17, 1979, nowU.S. Pat. No. 4,372,037, which is a continuation of application Ser. No.774,298, filed Mar. 4, 1977, in turn, a continuation-in-part ofcopending application Ser. No. 994,787 filed Mar. 3, 1975. The lattertwo applications now abandoned.

BACKGROUND OF THE INVENTION.

1. Field of the Invention

The present invention relates to means and method for providing largearea hybrid electronic packaging on a single substrate.

2. Description of the Prior Art

Current hybrid technology generally consists of fabricating andassembling passive and active components in a single small enclosurewhich is usually made hermetic. Examples of such enclosures are ceramicand metal packages which are 1/4" square or 1" having 14 or 30 leads.Thick and thin film techniques are utilized for producing passivecircuit elements on substrates mounted within these packages, anduncased discrete devices are used for semiconductors and other circuitelements. The hybrid microcircuit thus produced is typically theelectronic equivalent of an assembled printed wire board. The hybrid isultimately mounted onto a printed wiring board along with other hybridsand discrete components. Accordingly, the hybrid may be said torepresent the first level of integration and the assembled printedwiring board as the second level of integration. In such hybridtechnology utilizing assembled printed wiring boards, most thick filmhybrid work is restricted to substrates smaller than two inches squarewith very few multilayer techniques extending beyond two or threeconductor levels.

Examples of some prior art techniques are described in the followingpublications: "Low Cost Packaging of Thick-Film Substrates With CeramicGlass Seals" by J. C. Gioia, NEPCON PROCEEDINGS, 1970, Pages 10-42 to10-46; "Aspects of Multilayered Thick-Film Hybrids" by R. G. Loasby,Solid State Technology, May 1971, Pages 33-37, 46; "Evaluation Testingof a Thick Film Multilayer Interconnect System" by M. W. Rossman,INTERNATIONAL HYBRID MICROELECTRONICS SYMPOSIUM, 1970, pages6.3.1-6.3.9; "Fabrication of Multilayer Thick Film MicroelectronicCircuits" by L. K. Keys, A. J. Russo, F. J. Francis and S. Herring, Jr.in INTERNATIONAL HYBRID MICROELECTRONIC SYMPOSIUM, 1970, Pages6.4.1-6.4.10; and "Development of Large Thick-Film MultilayerAssemblies" by N. R. Isaak, J. W. Kans and E. G. Babiracki in 1971INTERNATIONAL MICROELECTRONICS SYMPOSIUM, Oct. 11-13, 1971, pages 1-27.Selected United States patents describing the prior art include U.S.Pat. Nos. 3,264,402; 3,302,067; 3,546,776; 3,581,375; 3,646,399;3,674,914; and 3,691,628.

Prior art hermetically sealed packages fail to meet the requirements ofa large area hybrid from several aspects. Four specific examples ofthese are discussed below. U.S. Pat. No. 3,609,294 speaks of thin filmsputtering and etching, rather than thick film screened and firedcircuits. It does not have a selective area hermetic seal nor is thedisclosed device a large area structure. U.S. Pat. No. 3,602,634 speaksof a simple small semiconductor package, not a hybrid microcircuit withselective area seals. The cover is sealed by brazing rather thansoldering. The insulating layers are of glass with deposited thin filmaluminum conductors rather than a thick ceramic dielectric with thickfilm gold or platinum-gold screened and fired conductors. U.S. Pat. No.3,753,054 discusses a large hermetic package designed to accommodate anLSI wafer, not a hybrid microcircuit. Hermeticity is obtained by brazingand welding rather than soldering and thus is not repairable. U.S. Pat.No. 3,673,309 discusses a single chip enclosure, rather than a hybridmicrocircuit. The cover is brazed (not soft soldered) directly to thesubstrate rather than to an intermediate ring frame. In addition, itseals to a gold paste, which experiences leaching and scavengingproblems, rather than to a platinum-gold alloy paste.

In general, the majority of prior art suffer from one basic respect.They are generally applicable only for small packages (usually less thanone-inch on a side), and not to large structures--as large as 6-incheson a side. Processes and materials which work on small structures cannotsimply be applied when the structure is expanded in area. In part, thelargest problem to overcome is one of finding the proper combination ofmaterials and processes which would work on large area structures whilestill maintaining the necessary reliability and yield.

The above-described prior art further suffers from one or more problems.The conventional printed wiring board design precludes the mosteffective use of board area and height required by the attachment ofhybrid packages and other electronic components to the printed wiringboard either directly or by wire bonds. Such a layout further results inextra weight which, for airborne and other uses, may result in a severepenalty in terms of space and added fuel consumption. The need tomechanically and electrically couple all individual components to theprinted wiring board results in a large number of interconnections withincreased possibility for resulting open and short circuits. Inaddition, because of the requirement to make a large number ofinterconnections, the complexity of each printed wiring board islimited. As a further problem, many components must be thermally coupledto a heat sink because of their high power loads. In general,conventional hybrid packages are mounted on a thermal supportingstructure which, in turn, must be thermally coupled to a thermaldissipation means, that is, a heat sink. Because of the number ofinterfaces due primarily to the various adhesives used for bondingpurposes in microcircuit fabrication, high thermal impedance occurs withthe result that the requirement to cool the printed wiring boardelectronic system presents a limitation as to how much power can beachieved with each system.

Repair of such printed wiring board systems is relatively complicated.For example, if an individual hybrid package exhibits some fault, thepackage leads must be unsoldered and the package removed from theprinted wiring board for replacement or repair. In the event of repair,the conventional package lid must be removed by heat applied to thesealing areas. Such heat may deleteriously affect otherwise functioningcomponents within the hybrid package, thereby leading to further wasteof material and components, in addition to loss and excessive use ofmanpower.

SUMMARY OF THE INVENTION

The present invention overcomes and avoids these and other problems bydirectly replacing the conventional assembled printed wiring board witha heat conductive base, such as of ceramic, on or within which isdirectly placed all components. Where, for example, the prior art wiringboard has several hybrid packages mounted thereon, the present inventionhybridises nearly the entire base or substrate, including all discretecomponents wherever possible. All connections are made by hybridtechnology, and selected area hermetic sealing of sensitive devices isprovided directly on the substrate. The substrate includes severallayers of electrical circuitry within and across its surface, andfurther includes thin and thick film resistors. Wherever possible,microelectronic chips are bonded directly to the substrate and to themultilayered circuitry, and only those chips and wire bonds requiringenvironmental protection are sealed, by a single enclosure with sides orpreferably by means of a ring frame seal secured to the substrate overinsulation with a cover or lid secured to the ring frame. Thus, ifrepair is required, only the cover need be removed and any heat producedthereby is sufficiently displaced from the chips by a distance greaterthan if removal were made at the substrate level.

While printed wiring board designs commonly include a large number ofdiscrete components, wherever possible the present invention utilizesequivalent chip versions of those components. However, if no equivalentexists, the discrete components can be soldered directly to thesubstrate.

In order to provide the necessary thermal dissipation, the presentinvention envisions a direct mounting of the substrate to a heat sinkstructure, whether comprising a cold plate or a honeycomb structure,such as is disclosed in copending application Ser. No. 554,789,entitled, "Thermally Controlled Electronic System Package" by Wesley E.Bartholomew, now U.S. Pat. No. 4,006,388, issued Feb. 1, 1977. By use ofsuch direct thermal mounting on a heat sink, a pair of substrates withcomponents mounted thereon, in accordance with the teachings of thepresent invention, may be mounted on one or both sides of the heat sinkfor economy of space and electronic design.

It is, therefore, an object of the present invention to provide for analternate approach to the conventional printed wiring board circuitdesign.

Another object is to provide for efficiency and savings in area overconventional techniques.

Another object is to provide for a decrease in weight per electricalfunction requirement.

Another object is to provide for fewer interconnections of components.

Another object is to provide for the ability to obtain more complexelectrical functions than the printed wiring board counterpart (i.e., ahigher packaging density).

Another object is to provide for greater flexibility in design andpartitioning of the electrical components and circuitry on a substrate.

Another object is to provide for decreased costs in testing andtrouble-shooting.

Another object is to provide for improved electrical performance byshortened conductor paths and by mounting components more closelytogether on the substrate.

Another object is to provide for improved reliability in view of thereduced number of interconnections, smaller size for a given electricalfunction, commonality of materials, and more uniform thermalenvironment.

Other aims and objects as well as a more complete understanding of thepresent invention will appear from the following explanation ofexemplary embodiments and the accompanying drawings thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts, in partial cross-section and in partialexplosion, the various concepts of the present invention; and

FIGS. 2a-2f depict the various steps, in elevational cross-section, ofthe techniques for forming the selective area seals depicted in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a large area hybrid microcircuit 10 comprises alarge area substrate 12 bonded, such as by brazing or by any othersuitable means, to a mounting plate 14. On substrate 12 are a pluralityof components such as semiconductor chips 16, capacitor chips 18,discrete components 20 and 21, such as special type resistors orcapacitors, thick film resistors 22, multilayer thick film conductors24, and environmentally protected components comprising any of the aboveas well as thin film substrates 25, residing within hermetic sealingareas 48. Sealing may be effected by an enclosure lid 64 directly bondedto sealing areas 48 or by a two part ring frame and cover described ingreater detail with respect to FIGS. 2a-2f.

The various components, generally identified by indicia 16-25, are allelectrically interconnected by appropriate circuitry 42 and extend tothick film solder pads 28, which are electrically coupled to terminals30 or to a pin or socket connector 32 by means of jumper wires 34 orother appropriate connections for electrically coupling large areahybrid microcircuit 10 into some larger electrical function.

As shown, mounting plate 14 comprises a solid block of material whichmay be of metal (such as aluminum or copper) or epoxy-glass laminate.The mounting plate serves both as a structural support and as a heatsink. If the substrate is thick enough (i.e., greater than 0.040-inch)and if the power requirements are low enough, no mounting plate isneeded. Mounting plate 14 may take the form of a heat sink configured asa cooling plate or cooling fins of corrugated thin stock, such as morefully described in above-noted U.S. Pat. No. 4,006,388, entitled"Thermally Controlled Electronic System Package", by Wesley E.Bartholomew. Regardless of the particular cooling method, the presentinvention is compatible with both forced-air and conduction coolingtechniques. In the forced-air cooled version, the preferableconstruction provides direct bonding of substrate 12 to the thin stock,such as described in U.S. Pat. No. 4,006,388. In the conduction cooledversion, there are two variations comprising conduction through thelarge hybrid ceramic substrate, and enhanced conduction by means of anattached metal heat sink.

Studies show that the forced-air cooled version produces a very lowthermal impedance, which is far lower than a printed wiring board with acopper heat sink, thereby making this version suitable for high powerapplications. The conduction cooled version with aluminium heat sink isalso comparable to or better than the printed wiring board with a copperheat sink. Without a heat sink, the conduction version has a relativelyhigh thermal impedance and should be used only for low powerapplications. When connected to a heat sink, the present invention hasan inherent thermal advantage compared to conventional hybrids mountedon a thermal supporting structure, due primarily to the fewer highimpedance interfaces between the heat source and the point of heatrejection. High impedance interfaces are due primarily to the variousadhesives used is microcircuit fabrication.

The particular construction of substrate 12, with its multilayer wiringand the components thereon as well as the method for preparing the same,may be appreciated with respect to FIG. 1, with a specific furtherfabrication technique and description of the structure later given withrespect to FIGS. 2a-2f. As shown, a structural support, such as mountingplate 14, is prepared with a plurality of connector receiving points 28thereon. As stated before, support 14 may be of any convenient material,such as of metal or a glass-base epoxy laminate. If, however, thesupport comprises cooling fins, connector receiving points arepreferably mounted directly on substrate 12.

Basic thick film substrate 12, preferably made of a ceramic materialsuch as 96% alumina, is prepared for processing thereof into a completedmicrocircuit. The surface of the substrate is first cleaned and thenmultilevel resistor, insulating or dielectric, and conductor patternsare screened and fired thereon. Firing of the conductors and resistorsis normally done in air at temperatures ranging from 800° C. to 1,000°C. using a conveyor furnace. The insulating or dielectric patterns maybe fired from 500° C. to 1,000° C. depending upon the material selected.These patterns may be provided in any suitable manner such as bymultilayering, crossover, or a combination thereof, along with multipleselective seal areas and thick film printing. In the multilayertechnique, conductive and dielectric patterns are alternately screenedon the substrate during its production utilising via holes therein so asto build up the conductor pattern within the substrate to a plurality oflevels. The via holes in the dielectric layers are used to electricallyinterconnect the adjacent conductor layers. In the crossover design,dielectric solder dams are used to prevent solder from touching theconductors and dielectric crossover paths, such as path 40, are placedon conductors over which upper level conductors are to extend, in themanner disclosed in copending Patent Application, Ser. No. 403,709,filed Oct. 4, 1973, now abandoned, entitled "Thick and Thin Multilayerand Crossover Process for Hybrid Microcircuits and Devices Made Thereby"by Frank Z. Keister and Robert Y. Scapple, two of the inventors herein.

This processing forms a circuit pattern, generally indicated by indicium42, comprising top level thick film conductors 44 and submerged or lowerlevel thick film conductors 46. Thereafter, all resistors are trimmed asrequired. Included within this screening and firing process is apreparation of selected seal areas 48 designed for hermetic sealing ofcomponents therein.

Although the complete fabrication of the selective area hermetic sealswill be described in greater detail hereinafter, in brief, a metal ringframe 72 is secured to area 48, such as by soldering, followed by a leaktest to determine the hermeticity of the seal.

Thereafter, all conductor paths which are eventually used for solderingare pretinned. Substrate 12 is then adhesive-bonded to structuralsupport 14, such as by a silicon or epoxy adhesive, or brazed to acooling fin. Discrete components 20 and 21, which do not have a chipsize equivalent, are then soldered to substrate 12, with the discretecomponents being adhered in place by means of an epoxy or polysulfaneresin prior to soldering. All strap wires are then soldered to thesubstrate and from solder pads 28 to connection points, such asterminals 30, on support 14 or directly to pins 33 of a connector, suchas connector 32.

Chip components 16 (such as transistor chips, diode chips, or capacitorchips) are then mounted within ring frame 72 and wire bonded to theterminal bonding points 56 which extend from circuit pattern 42 outsideof selected sealing areas 48 to within the selected sealing areas.

A connector 32 is then adhesive-bonded or otherwise mounted and solderedto structural support 14 with leads 34 therefrom bonded to thick filmsolder paths 28 on substrate 12. Certain designs of large area hybridsdo not necessarily use connectors. Other means such as contour cablesare then used to electrically couple substrate 12 to the next highercircuit function. An epoxy adhesive is then placed over all the solderedjoints, if required, and presealing electrical tests are conducted. Anytrimming of thick film resistors may be made by laser trimming methods,followed by any rework as may be necessary. Thereafter, a cover, such ascovers 64 and 65, in which cover 65 comprises integral flanged sides 65aand top 65b and cover 64 is without flanges, or as described withrespect to FIGS. 2a-2f, is soldered to ring frame 72 in a dry box and ina nitrogen atmosphere. FIG. 1 shows covers 64 and 65 being solereddirectly to the seal areas 48 and 25 on the substrate, whereas cover 80is soldered to ring frame 72. Both solder seals are applicable, however,the ring frame is utilized where repair is contemplated should thehybrid malfunction. Both fine and gross leak tests for hermeticity areconducted followed by acceptance testing. For added protection andconfidence, the entire large area hybrid microcircuit may be thenconformally coated with a polyurethane plastic.

Referring now to FIGS. 2a-2f, the structure and steps for making theselective area hermetic seals for sealing areas 48 will be described.Starting with the thick film screened and fired sealed area 48, such asdepicted in FIGS. 1 and 2a, the seal area is covered with a dielectricclosed loop 66 for insulating any leads 44 of the underlying circuitpattern 42. The dielectric material is a glass-ceramic paste which isscreened on and subsequently fired at temperatures ranging from 500° to1,000° C. depending upon the material. Two dielectric screenings arenormally used in order to build up a sufficient thickness to precludepinholes. Dielectric 66 prevents electric shorting of metal loop 68 andring frame 72 to underlying conductor 42. The underlying pattern 42 isusually of a conductive thick film paste, such as gold or platinum-goldwhich is wire-bondable.

Over dielectric closed loop 66 is placed a metal closed loop 68, such asa thick film paste of a platinum-gold alloy, or a platinum-silver alloy.The metal loop 68 is screened and fired at temperatures from 800°C.-1,000° C. The metal loop must be of a material which is solderable,non-leaching and non-scavenging and must be compatible with thedielectric over which it is screened. Thereafter, metal closed loop orsealing area 68 is then tinned with an appropriate solder 70. The solderused must be ductile enough to prevent any future fracture or leakage ofthe hermetic seal, it must wet but not scavenge or leach the thick filmpaste beneath it, and it must be strong enough to withstand the rigorsand stresses of repeated temperature cycling, thermal shock, and hightemperature storage at temperatures up to +150° C. Tests have shown thatSn10 solder (10% tin, 90% lead) is satisfactory and that other solders,such as Sn96, Sn63, and Sn60 are not. A ring frame 72, preferably ofnickel-gold plated KOVAR (an iron-nickel-cobalt alloy), having an Sn10pretinned surface 74 is sealed to tinned area 70, such as by solderreflow techniques, see FIGS. 2b and 2c. Thereafter (FIG. 2d), an uppersurface 76 of ring frame 72 is tinned with a solder 78. The solder 78must match the solder which will eventually be used to seal the cover tothe ring frame. A solder alloy known as Sn96 (96% tin, 4% silver) issuitable. As shown in FIG. 2e, a cover 80, preferably also of KOVARwhich has been nickel plated and tin plated, although cold rolled steelmay be used, and preferably having a flange 82 at its periphery, whichis tinned, is then solder sealed to ring frame 72 at its tinned uppersurface 76. The sealing solder 78 is an Sn96 alloy. This solder mustform a hermetic seal, must withstand thermal and mechanical shock, andrepeated temperature cycles from -65° C. to +150° C., must wet and forma smooth solder fillet with the cover and ring frame, and should have amelting point lower than that of the solder used to join the ring frameto the substrate. Sn96 solder melts at 221° C. which is 78° C. lowerthan the liquidus point of Sn10 solder (299° C.). Sealing solder 78 mustalso permit removal and replacement of KOVAR cover 80 without degradingring frame 72. Sealing of the cover 80 to the ring frame 72 is done in adry box (preferably at a dew point of -55° C.) in a nitrogen atmospherewithout the use of flux. Hermetic sealing of the enclosure may beprovided by a hole 84 in cover 80 which is thereafter closed with solder86, as seen in FIG. 2f; however, hole 84 may be dispensed with if propersealing is otherwise obtained. Leak tests are made preferably at eachstep in the process.

Although cover 80 may be solder sealed directly to substrate 12 withoutthe need for ring frame 72, it is preferred to use the ring frame inorder to enhance repairability. For repair, only the cover need beremoved with maximum protection against accidental damage to any of thecomponents contained within the sealing area. Furthermmore, if it isdesired, cover 80 need not be flanged but may comprise a flat cover orone having simply turned-down edges; however, it has been found mostpractical to utilize a flanged cover. In addition, a raised cover (asopposed to a flat cover) provides added height clearance within thesealed area which may be required for some large components and wirebond loops.

A unique feature of the hermetic seals (FIGS. 2a-2f) is that they can bemade at preselected areas on a larger substrate. Such seals may be of avariety of sizes, such as 0.6-inch in diameter, 1×1 inch, or 1×2-inch.Another unique feature of these seals is that thick film conductors maybe routed beneath the perimeters of the sealed areas as a means ofgetting electrical signals into and out of the hermetic area. Stillanother unique feature of these seals is that they are repairable andmay be of a large size. Seals as large as 3-inch diameter have been madeusing the materials and processes described in this invention.

Although the invention has been described with reference to particularembodiments thereof, it should be realised that various changes andmodifications may be made therein without departing from spirit andscope of the invention.

What is claimed is:
 1. A large area hybrid microcircuit assembly forperforming an electrical function comprising:a thermally conductivesubstrate of electrically insulating material; a plurality of thick filmmultilayered electrical leads extending within and across the surface ofsaid substrate for forming an interconnecting electrical circuit; aplurality of electronic components secured directly to said substrateand electrically coupled to said interconnecting electrical circuit foreffecting said electrical function; and means defining thick filmhermetic seals at one or more selected areas directly on said substrateand enclosing selected ones of said electronic components for protectingsaid selected electronic components from the environment surroundingsaid selected areas, said hermetic seals means further comprising,meansdefining a closed loop of thick film dielectric material placed overinput, output and ground leads extending from said interconnectingelectrical circuit and terminating within each of said selected areasand electrically coupled to at least one of said selected electroniccomponents, means defining a closed loop of thick film sealing metalplaced over said dielectric material closed loop means and consisting ofsolderable material which is compatible with said dielectric material,means defining a solder on said sealing metal closed loop means andconsisting of a material sufficiently ductile for militating againstfracture and leakage of portions of said hermetic seals, andmeansdefining an enclosure cover having sides sealed to said sealing metalclosed loop means by said solder means, comprising means defining aclosed ring frame sealed to said sealing metal closed loop by saidsolder means and having a sealing surface extending above said selectedelectronic components within said selcted area; means defining a secondsolder on said closed ring frame means and consisting of a materialcapable of forming a hermetic seal and of withstanding thermal andmechanical shock and repeated temperature cycles such as from -65° to150° C., with a melting point lower than that of said ring frame tometal closed loop solder means; and means defining a cover sealed tosaid sealing surface of said closed ring frame means by said secondsolder means.
 2. A microcircuit assembly as in claim 1 wherein saidsealing metal closed loop means comprises one of a thick filmplatinum-silver alloy paste, or a platinum-gold alloy, and tinned withSn10 solder alloy.